Cardiac pacing electrically stimulates the heart when the heart's natural pacemaker and/or conduction system fails to provide synchronized atrial and ventricular contractions at appropriate rates and intervals for a patient's needs. Such bradycardia pacing provides relief from symptoms and even life support for hundreds of thousands of patients. Cardiac pacing may also give electrical overdrive stimulation intended to suppress or convert tachyarrhythmias, again supplying relief from symptoms and preventing or terminating arrhythmias that could lead to sudden cardiac death.
Cardiac pacing is usually performed by a pulse generator implanted subcutaneously or sub-muscularly in or near a patient's pectoral region. The generator usually connects to the proximal end of one or more implanted leads, the distal end of which contains one or more electrodes for positioning adjacent to the inside or outside wall of a cardiac chamber. The leads have an insulated electrical conductor or conductors for connecting the pulse generator to electrodes in the heart. Such electrode leads typically have lengths of 50 to 70 centimeters.
Known pulse generators can include various sensors for estimating metabolic demand, to enable an increase in pacing rate proportional and appropriate for the level of exercise. The function is usually known as rate-responsive pacing. For example, an accelerometer can measure body motion and indicate activity level. A pressure transducer in the heart can sense the timing between opening and closing of various cardiac valves, or can give a measure of intracardiac pressure directly, both of which change with changing stroke volume. Stroke volume increases with increased activity level. A temperature sensor can detect changes in a patient's blood temperature, which varies based on activity level. The pacemaker can increase rate proportional to a detected increase in activity.
Pulse generator parameters are usually interrogated and modified by a programming device outside the body, via a loosely-coupled transformer with one inductance within the body and another outside, or via electromagnetic radiation with one antenna within the body and another outside.
Although more than five hundred thousand pacemakers are implanted annually, various well-known difficulties are present.
The pulse generator, when located subcutaneously, presents a bulge in the skin that patients can find unsightly or unpleasant. Patients can manipulate or “twiddle” the device. Even without persistent twiddling, subcutaneous pulse generators can exhibit erosion, extrusion, infection, and disconnection, insulation damage, or conductor breakage at the wire leads. Although sub-muscular or abdominal placement can address some of concerns, such placement involves a more difficult surgical procedure for implantation and adjustment, which can prolong patient recovery.
A conventional pulse generator, whether pectoral or abdominal, has an interface for connection to and disconnection from the electrode leads that carry signals to and from the heart. Usually at least one male connector molding has at least one terminal pin at the proximal end of the electrode lead. The at least one male connector mates with at least one corresponding female connector molding and terminal block within the connector molding at the pulse generator. Usually a setscrew is threaded in at least one terminal block per electrode lead to secure the connection electrically and mechanically. One or more O-rings usually are also supplied to help maintain electrical isolation between the connector moldings. A setscrew cap or slotted cover is typically included to provide electrical insulation of the setscrew. The complex connection between connectors and leads provides multiple opportunities for malfunction.
For example, failure to introduce the lead pin completely into the terminal block can prevent proper connection between the generator and electrode.
Failure to insert a screwdriver correctly through the setscrew slot, causing damage to the slot and subsequent insulation failure.
Failure to engage the screwdriver correctly in the setscrew can cause damage to the setscrew and preventing proper connection.
Failure to tighten the setscrew adequately also can prevent proper connection between the generator and electrode, however over-tightening of the setscrew can cause damage to the setscrew, terminal block, or lead pin, and prevent disconnection if necessary for maintenance.
Fluid leakage between the lead and generator connector moldings, or at the setscrew cover, can prevent proper electrical isolation.
Insulation or conductor breakage at a mechanical stress concentration point where the lead leaves the generator can also cause failure.
Inadvertent mechanical damage to the attachment of the connector molding to the generator can result in leakage or even detachment of the molding.
Inadvertent mechanical damage to the attachment of the connector molding to the lead body, or of the terminal pin to the lead conductor, can result in leakage, an open-circuit condition, or even detachment of the terminal pin and/or molding.
The lead body can be cut inadvertently during surgery by a tool, or cut after surgery by repeated stress on a ligature used to hold the lead body in position. Repeated movement for hundreds of millions of cardiac cycles can cause lead conductor breakage or insulation damage anywhere along the lead body.
Although leads are available commercially in various lengths, in some conditions excess lead length in a patient exists and is to be managed. Usually the excess lead is coiled near the pulse generator. Repeated abrasion between the lead body and the generator due to lead coiling can result in insulation damage to the lead.
Friction of the lead against the clavicle and the first rib, known as subclavian crush, can result in damage to the lead.
In many applications, such as dual-chamber pacing, multiple leads can be implanted in the same patient and sometimes in the same vessel. Abrasion between the leads for hundreds of millions of cardiac cycles can cause insulation breakdown or even conductor failure.
Data stored in memory of implanted pulse generators is typically made available to a physician or other personnel for collection and/or analysis. For example, information is sought regarding system performance and trouble-shooting relating to the device, lead system, and/or patient in an acute, clinical setting. The information is generally supplied via a telemetry capability between the external programmer and the implanted device. In addition, an external programmer can be used to adjust parameters of multi-function implantable medical devices, such as pacing rate, pulse amplitude, sensed signal gain, and pulse timing and coordination.
Typically, an external programmer used during a telemetry procedure is positioned remotely from the patient. A programming head of the programmer such as a wand or other external device, containing an antenna or coil, is connected to the remainder of the programmer via a stretchable coil cable and is positioned over the patient's implanted device site for programming or telemetry interrogation of the implanted device.
Communication between the implanted medical device and the external programmer is facilitated by receiving and transmitting circuitry included within the implanted medical device and external programmer. Bandwidth is generally kept low to minimize power consumed by the implanted medical device. Power consumption is a consideration in designing implantable medical devices since the devices are typically powered by a depletable energy source, such as a primary battery. Replacement of an implanted medical device due to battery depletion can be costly and inconvenient.
Therefore, minimization of power consumption by the implanted medical device is a design and operational consideration. To facilitate power consumption management, transmitter and receiver circuitry can be powered down when not in use but are to be awakened when desired to enable communication. Awakening can occur periodically, in which the implantable device checks for a communication signal at regular intervals. The awakening process can otherwise be achieved by using electromagnetic energy coupled to the receiving antenna or coil to facilitate the wake up function. Awakening techniques result in a complicated telemetry protocol, which generally results in longer linkup times. In addition, the awakening techniques employ a relatively large antenna or coil, which is undesirable and inconsistent with a physically compact implanted medical device.
In addition to power reduction and small size, another design criterion for implanted medical devices is accurate communication of data. Communication often occurs in environments such as hospitals and doctors' offices, which can be noisy due to the presence of other electronic and electromagnetic sources. To achieve robustness of the link, bandwidth is generally kept low, with small packet sizes. To assure that data are transmitted accurately, the antenna or coil in the implantable device is typically positioned to maximize signal strength, both transmitted and received.